Some prior art heat sealing machines for bags, for example, used heated conductor bars which pressed against or were very closely adjacent the bag material, for example, kraft paper outer ply with a heat sealable inner ply such as polyethylene film. A typical construction was a machine about 60 feet long with many separate heaters in rod form placed along the path of movement of the bag forms. Such machines were inefficient in transfer of heat through the kraft paper to the PE film and also had a large heat storage capacity so that they could not respond quickly to require the changes in heat. Many such electrical bar heaters were of the sheathed type wherein an electrical resistance wire is separated from a tubular metal sheath by electrical insulation. This electrical insulation is also generally a heat insulation so that the transfer of heat from the inner electrical resistance wire to the outer tubular metal sheath was not a rapid process. Next the heat had to be transferred by radiation or conduction through the sheath to the kraft paper and then by conduction through the paper to the PE film. If there was a good surface contact between the rod heaters and the moving bags, then there was a frictional drag at this point which often marked the bags to make them unsightly. If there was a very small space between the bag and the heater, then the transfer of heat was greatly impeded. One result was that the entire sealing machine then became quite hot and personnel had to be shielded from this excess heat. Another defect was the slowness of operation of the machines and a further defect was the difficulty of locating burned out heater elements in the long machine. Where the sealing machine was used in the construction of bags to seal one end of the bag, a rate of 60 bags per minute was a usual production rate.
Such machines, however, were in operation for sealing the inner ply of a two ply bag where the outer ply was of kraft paper, because enough heat could be imparted through the kraft paper to heat seal the inner PE ply. However, present day usage of heat sealed bags has increased and ever tougher bags are desired, especially those which will handle chemicals, foodstuffs and other fine powdery material without leakage and also be vermin free, yet be tough enough for domestic shipment and shipment to foreign countries and withstand the rough handling in route. Such bags have been constructed and sometimes have as many as three kraft paper plies plus a tough plastic reinforcing ply plus a heat sealable inner ply. These may be termed barrier bags, reinforced bags or pouch bags.
Other recently developed bags which are quite difficult to seal are those which are lacquer coated on the exterior surface, because such lacquer becomes discolored or disfigured by the heater bars of the sealing machine. Other bags which are difficult to seal are those which are gravure printed on a clay coated bleached kraft outer ply. Such current methods of heat sealing damage the appearance of such special bags. Still other bags difficult to seal with bar type heaters are those which use a reinforcing ply which is formed of two cross laminated oriented plastic films sold under the trademark Valeron. This has a cross grained structure similar to plywood for strength, yet the strength of the heat sealable Valeron film is greatly weakened by bar type heaters where the heat is conducted through the kraft paper plies and Valeron ply to the inner heat sealable ply.